Test script was using sleep to try and catch an OSD in the middle of a backfill, sometimes the OSD was too fast and completed the backfill before the script could query the state of the OSD.

Fix the script to use the backfill_toofull error inject to block the backfill from completing so the script can test the acting set is correctly before removing the error inject and allowing the backfill to complete

Fixes: https://tracker.ceph.com/issues/70949
Fixes: https://tracker.ceph.com/issues/71071

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